The invention concerns the crowns of tires and, more particularly, the rubber bondings in contact with the reinforcing cords of those crowns.
The crowns of tires usually comprise a carcass reinforcement, a belt reinforcement with usually at least two superposed reinforcing plies formed by cords parallel in each ply and criss-crossed from one ply to the other and a tread.
It is well known that under harsh conditions of use, as the distances covered increase, fissures can appear at the lateral end of the cords of the crown reinforcing plies. Those fissures can then be propagated along the cords and be joined together between two or more cords and thus result in a separation of the lateral ends of the two crown reinforcing plies.
To limit the initiation and propagation of those fissures, bands of rubbery compound of lower modulus of elasticity than that of the rubber bonding layers of the crown plies are usually inserted between the ends of the reinforcing plies.
The object of the invention is a tire whose crown structure is capable of containing the phenomena of separation of the lateral ends of the crown reinforcing plies and of reducing their appearance to stress levels affording an even greater margin of safety.
In what follows, xe2x80x9ccordxe2x80x9d is understood to mean monofilaments as well as multifilaments, or assemblages like cables, yarns or even any type of equivalent assemblage, whatever the material and treatment of those cords, the term cord embracing any type of surface covering or coating, notably of rubber having undergone the start of vulcanization or pre-sizing to promote adherence to the unvulcanized rubber.
xe2x80x9cRubber bonding layerxe2x80x9d for a given reinforcing ply is understood to mean the rubbery compound in contact with the ply reinforcing cords, adhering to the latter and filling the interstices between adjacent cords. In current industrial practice, for a given crown or carcass reinforcing ply, the same rubber bonding composition is used for the different zones of the tire.
xe2x80x9cContactxe2x80x9d between a cord and a rubber bonding layer is understood to mean that at least one part of the outer circumference of the cord is in close contact with the rubbery compound constituting the rubber bonding. If the cord contains a covering or coating, the term contact means that the outer circumference of that covering or coating is in close contact with the rubbery compound constituting the rubber bonding.
xe2x80x9cModulus of elasticityxe2x80x9d of a rubbery compound is understood to mean a secant extension modulus obtained on a uniaxial extension deformation in the order of 100% after two cycles of accommodation to that deformation amplitude and at ambient temperature.
The tire according to the invention comprises a crown, two sidewalls and two beads, a carcass reinforcement anchored in the two beads and a belt reinforcement, the belt reinforcement comprising at least two superposed reinforcing plies formed by cords parallel in each ply and criss-crossed from one ply to the other by forming angles (xcex1, xcex2) with the circumferential direction ranging between 10xc2x0 and 55xc2x0. The tire is characterized in that the cords of at least one of the two reinforcing plies are successively in contact, on the same side of said ply, by axially following the equatorial plane of the tire to at least one of the lateral ends of the reinforcing ply, with at least one rubber bonding layer of a first modulus of elasticity and then a rubber bonding layer of a second modulus of elasticity less than the first.
The rubber bonding layer of modulus of elasticity less than that of the rubber bonding layer placed in the central part of the crown has the advantage of better withstanding fissuring in the lateral end zones of markedly stressed reinforcing plies.
According to a first embodiment, the axial widths of the two crown reinforcing plies being different, the cords in contact with the rubber bonding layers of a first and a second modulus of elasticity are the cords of the reinforcing ply whose axial width L is smallest and the said rubber bonding layers of a first and a second modulus of elasticity are placed radially on the side of the second crown reinforcing ply.
That embodiment has the advantaging of appreciably diminishing the maximum shear stresses between the two plies, which is very favorable for limiting fissuring phenomena.
According to a second embodiment, the rubber bonding layers of a first and a second modulus of elasticity are placed radially between the two crown reinforcing plies and are each in contact with the cords of the two reinforcing plies.
That embodiment has the advantage of limiting to one the number of rubber bonding layers placed radially between the two reinforcing plies, which facilitates manufacture of the tire.
The cords of at least one of the axial ends of the crown reinforcing ply whose axial width is smallest are preferably in contact, radially on both sides of said ply, with the said rubber bonding layer of a second modulus of elasticity.
The cords of the axial ends of the two crown reinforcing plies, placed at least on a same side of the equatorial plane, can also be in contact, radially on both sides of those plies, with the rubber bonding layer of a second modulus of elasticity.
A rubber decoupling layer of a third modulus of elasticity can, furthermore, be placed between the axial ends of the said crown reinforcing plies, on one side at least of the equatorial plane and in which the third modulus of elasticity is less than the second modulus of elasticity.
Those variants strengthen the resistance to fissuring at the end of the cords of the crown plies.
The axial width of the zones of contact between a rubber bonding layer of a second modulus of elasticity and the cords of a reinforcing ply is greater than 5% of the axial half-width of the reinforcing ply. It is not necessary for it to be greater than 30% of that axial half-width.
In addition, the cords of the radially outer reinforcing ply are in contact, at least in the center zone of that ply, on the radially outer side, with a rubber bonding layer of a fourth modulus of elasticity. The fourth modulus of elasticity is less than the first modulus of elasticity.
That arrangement is particularly advantageous when the belt reinforcement comprises a third reinforcing ply placed radially outside the two crossed reinforcing plies. That third reinforcing ply can consist of reinforcing plies oriented in the circumferential direction of the tire.
The ratio between the second modulus of elasticity and the first modulus of elasticity ranges between 0.5 and 0.9 and preferably between 0.6 and 0.8. The first modulus of elasticity can range between 9 and 13 MPa.
The third modulus of elasticity can range between 3 and 6 MPa and the fourth between 3 and 8 MPa.
According to one advantageous arrangement, the rubber bonding layers extend axially outward beyond the axial end of the cords of the crown plies.
According to another embodiment, the cords of at least one of the reinforcing plies are in contact, in the center part of the crown, with an additional rubber bonding layer of a fifth modulus of elasticity higher than the first modulus of elasticity. That fifth modulus of elasticity can range between 12 and 20 MPa.
This embodiment is particularly advantageous when the belt reinforcement does not contain any reinforcing ply composed of cords oriented in the circumferential direction of the tire. In that case, the rigidity of the rubber bondings in the center part of the crown contributes appreciably to the overall rigidity of the crown and contributes to obtaining excellent road performance characteristics of vehicles equipped with such tires.
Advantageously, the cords of one of the crown plies having a diameter xcfx86, any rubber bonding layer of a second modulus of elasticity in contact with those cords has a thickness opposite the cords, at least in the lateral end zone of that crown ply, greater than or equal to xcfx86/2. That minimum thickness appreciably improves resistance to fissuring of the crown of tires according to the invention. In the case of vehicles subjected to intense running stresses, it is desirable for that minimum thickness to be greater than or equal to xcfx86.